Oil well pipes such as tubing which a fluid such as crude oil flows inside or casing which is surrounding the tubing are used in the drilling of oil wells for extraction of crude oil or gas oil. The oil well pipes are normally connected (fastened) to each other using threaded connections. In the past, the depth of oil wells was between 2,000 and 3,000 meters. However, in deep oil wells such as offshore oil fields of recent years, the depth may sometimes reach 8,000 to 10,000 meters. The length of an individual oil well pipe is typically ten-odd meters. Hence, in some cases a huge enormous number of 1000 oil well pipes or more are connected together.
In their usage environment, threaded connections for oil well pipes are subjected to loads such as axial tensile forces caused by the weight of the oil well pipes and the connections themselves, compounded pressure of internal and external interfacial pressure and the like, and geothermal heat. Therefore, the threaded connections for pipes or tubes need to be able to maintain airtightness without being damaged even in such a severe environment.
A pin-box structure constituted by a portion referred to as a pin that has an external thread and a portion referred to as a box that has an internal thread as described, for example, in Patent Literature 1 may be mentioned as a typical threaded connection for pipe or tube that is used to fasten oil well pipes. Typically, a pin is formed on both end portions of an oil well pipe, and a box is formed on the inner surface of both sides of a threaded connection component referred to as coupling.
During the process of lowering tubing or casing into an oil well, due to various reasons such as trouble arising, a threaded connection which was previously tightened is sometimes loosened, the threaded connection is temporarily lifted up from the oil well, and thereafter retightened and lowered into the well again. According to the API (American Petroleum Institute) standards, it is desirable that there is no occurrence of unrepairable seizing referred to as galling and for airtightness to be maintained even if tightening (make-up) and loosening (breakout) are performed ten times for a connection for tubing or three times for a connection for casing.
In order to increase galling resistance and airtightness when tightening threaded connections for oil well pipes, a lubricant referred to as compound grease is applied to contact surfaces of the threaded connections (that is, to thread parts and unthreaded metal contact parts). The term compound grease refers to a viscous liquid lubricant (a greasy lubricant) which contains heavy metal powders. An example of compound grease is prescribed in API BUL 5A2.
In the past, it has been proposed to carry out surface treatment to form one or more layers on the contact surfaces of threaded connections to increase the retentivity with respect to compound grease and improve sliding properties. Examples of such surface treatment include nitriding, various types of plating including zinc plating and dispersion plating, and phosphate chemical treatment.
Compound grease contains a large amount of heavy metal powders such as zinc, lead, and copper. Hence it is pointed out that there is the possibility that a heavy metal will affect the use environment, for example, the marine environment, and the working environment.
In recent years, strict environmental restrictions are being enacted on a global scale. The use of compound grease is also already being restricted in some regions. Accordingly, to avoid effects on the environment and humans during work to drill gas wells and oil wells, a demand has developed for threaded connections which exhibit excellent galling resistance without using compound grease.
The present applicants proposed a threaded connection for pipe or tube that can be used for fastening oil well pipes without the application of compound grease in Patent Literature 2. In the threaded connection disclosed in Patent Literature 2, a contact surface of at least one of a pin and a box is coated with a two-layer coating consisting of a viscous liquid or semisolid lubricating coating and a dry solid coating formed thereon. The dry solid coating is formed from a thermosetting resin coating such as an acrylic resin or an ultraviolet curing resin coating. The viscous liquid or semisolid lubricating coating is tacky and consequently foreign matter easily adheres thereto. However, the tackiness is eliminated by forming the dry solid coating on the viscous liquid or semisolid lubricating coating. Since the dry solid coating is broken at the time of fastening a threaded connection, the dry solid coating does not impair the lubricity of the lubricating coating formed thereunder.
Further, in Patent Literature 3, the present applicants proposed a threaded connection in which a thin non-sticky lubricating coating is formed on thread parts (a pin and a box). The lubricating coating described in Patent Literature 3 is obtained by dispersing solid lubricant particles in a solid matrix exhibiting plastic or viscoplastic rheological behavior (flow properties). The fusing point of the solid matrix is preferably in the range of 80 to 320° C. The lubricating coating is formed by spray coating in a molten state (hot melt spraying), by flame spraying using a powder, or by spray coating of an aqueous emulsion. A composition used in the hot melt spraying contains, for example, polyethylene as a thermoplastic polymer, wax (for example, carnauba wax) and a metal soap (such as zinc stearate) as a lubricant component, and calcium sulfonate as a corrosion inhibitor.
Still further, in Patent Literature 4, the present applicants proposed a threaded connection for pipe or tube in which a solid anticorrosive coating is formed on a contact surface of a pin, and a solid lubricating coating is provided on a contact surface of a box. The solid anticorrosive coating includes an ultraviolet curing resin as a main ingredient, and preferably is transparent. The solid lubricating coating is preferably formed by hot melt spraying from a composition containing a thermoplastic polymer, wax, a metal soap, a corrosion inhibitor, a water-insoluble liquid resin and a solid lubricant, and exhibits plastic or viscoplastic rheological behavior.